This invention relates to monolithic multilayer capacitor structures, and more particularly, to such structures employed for high voltage applications. Additionally, this invention relates to minimizing generation of spurious signals in vibrating capacitors employing piezoelectric materials as dielectrics.
High voltage ceramic capacitors are widely available. These are generally constructed of electrode plates sandwiched around a dielectric formed of a ceramic material. The size and characteristics of the dielectric help determine the value of the capacitance, and high voltage capacitors rated in excess of 10 kV are very bulky, costly and limited in use because of their size. An example of such a high voltage ceramic capacitor is shown in U.S. Pat. No. 3,946,290.
Monolithic ceramic multiple electrode capacitor assemblies are widely used in the electronics industry. Such capacitors are either constructed as parallel or series assemblies. When parallel multiple electrode capacitor assemblies are formed, electrodes embedded in a monolithic medium extend inwardly from opposite edges with a dielectric material sandwiched between the opposite electrodes. By building alternate layers of such electrodes and connecting common electrodes at the side edges, a multiple electrode parallel capacitor assembly is formed.
In a series capacitor assembly, spaced-apart aligned electrodes extend inwardly from opposite edges toward each other and terminate in a gap. A floating electrode separated from the spaced-apart electrodes by a gap in the dielectric medium bridge the electrodes to form the series construction.
Such multiple electrode capacitor assemblies have their stress points at the edges of the electrodes as well as between electrode layers. The voltage rating of such capacitors is limited by this stress, which limits the dielectric withstanding voltage. For most dielectric materials, the dielectric withstanding voltage rating generally decreases per unit thickness with increasing dielectric thickness, but the overall total dielectric withstanding voltage increases. Therefore, relatively thick dielectrics are required for high voltage applications.
Prior art monolithic multiple electrode parallel capacitors also suffer from potential catastrophic breakdown when dielectric failure occurs between the electrodes. Since these capacitors are connected in parallel, such dielectric failure effectively destroys or severely alters the capacitor characteristics. These alterations tend to detract from the reliability of such capacitors.
Multiple electrode capacitor assemblies also suffer from practical design constraints which affect the ability to produce capacitors having predicted capacitance ratings. This is due to difficulty in accurately controlling dielectric thickness, electrode area and the dielectric characteristics which are influenced by the sintering process employed for monolithics.
Frequently, capacitor structures are formed by electrically joining individual capacitors. Such structures suffer from the inherent disadvantages attendant such assembly techniques in which a number of joints are employed, thereby detracting from the reliability of such assemblies.
It also has been found that unwanted spurious signals are generated in multi-layer type capacitors when such capacitors utilize piezoelectric material as the dielectric ceramic and the capacitor is subjected to vibratory action. These spurious signals occur because of the nature of the piezoelectric material. Since these capacitors may be used in aerospace applications, it can be understood that minimizing such spurious signals is important.
In an article entitled Piezoelectric Properties of Internally Electroded PZT Multilayer Capacitors appearing in the magazine Ferroelectrics, 1980, Volume 27, pp. 59-62, it has been posited that a pinning effect exists for parallel multielectrode construction using a piezoelectric as a dielectric. The pinning effect may occur at the edges of the electrodes in the parallel electrode capacitor construction.
An object of this invention is to provide a high voltage monolithic capacitor which is relatively small in size.
Another object of this invention is to provide a high voltage rated monolithic multiple electrode capacitor assembly.
Still another object of this invention is to provide such a capacitor construction in which the rated capacitance value is more effectively and accurately achieved.
Another object of this invention is to provide such a capacitor construction whose reliability is enhanced.
Still another object of this invention is to provide a multiple electrode capacitor assembly employing a piezoelectric dielectric in which the amount of vibratory movement of the piezoelectric dielectric is limited to reduce the emission of unwanted spurious signals generated by the vibrating action.
Other objects, advantages and features of this invention will become more apparent hereinafter.